What is the best website to look for public transportation between London and towns / cities such as Bath, Oxford, Cambridge, Windsor, Canterbury and Salisbury?
If you just want a general idea, Google Maps works well for that. Once you've gotten the basic info, you can move on to National Rail for detailed information.
The Nationalrail website listed above is the official website for UK trains.
Bus there are a couple of excllent bus systems in the UK:
and
These are luxury coaches and not city buses.